Peer to Peer (also known as P2P) exists as a configuration for utilizing networks communicating by direct exchange of information between an indeterminately large number of terminals. In recent years, the application of peer to peer technology to traffic systems has been considered. For example, if inter-vehicle communication, which is wireless communication between vehicles, is carried out, it is possible to transmit information such as the situation for one vehicle, the situation on the road the vehicle is traveling on, and the degree of traffic congestion surrounding the vehicle, to another vehicle in real time. Moreover, by communicating GPS information between vehicles, it is possible to make a driver of another vehicle aware of accurate information for the position and time of one vehicle. If a network is constructed using inter-vehicle communication at a place where there is congestion, there is no need to install expensive equipment for collecting congestion information at the road itself.
Wireless P2P is expected to take on the role of preventing accidents before they happen. If vehicles are approaching an intersection with no visibility, it is possible to alert drivers by sending information to the effect that “a vehicle is approaching the intersection” between the vehicles.
Two types of P2P technology exist, P2P technology requiring the mediation of a central server, and P2P technology where data is passed using a bucket brigade method (a multi-hop wireless network). Inter-vehicle communication is the latter multi-hop wireless network method. In the multi-hop wireless network method, even if communication is blocked at one location, the network as a whole does not stop. The multi-hop wireless network method does have the drawback, however, that network congestion is accelerated as the number of users increases.
Devices for transmitting and receiving signals between mobile units such as vehicles, such as, for example, disclosed in patent document 1 in the following, are well-known.
If the apparatus disclosed in patent document 1 is used, when one vehicle detects traffic conditions or anomalous events, this vehicle can send the detected information to all of the vehicles within wireless transmission range.
However, high-frequency signals are typically utilized in wireless communication used in inter-vehicle communication systems. It is therefore not possible for vehicles detecting information to communicate with other vehicles concealed by impediments to communication when impediments to communication such as, for example, buildings etc. that block signals are present.
On the other hand, multi-hop wireless network technology where communication between mobile units that cannot communicate with each other directly is made possible by a plurality of mobile units relaying signals is also well-known as mobile unit communication technology. A routing protocol for constructing a path for various multi-hop wireless networks has also been developed. Namely, communication is made possible even when signals cannot be transmitted directly between mobile units by relaying signals to other mobile units.
However, multi-hop wireless networks for mobile units have the drawback of slow convergence of path caused by disordered hopping so that communication efficiency is poor.
Methods for improving efficiency of communication between specific vehicles on multi-hop wireless communication networks are proposed in patent document 2 and patent document 3. However, these methods do not improve efficiency of communication of multi-hop wireless networks between indeterminate mobile units.
In patent document 4, technology is disclosed capable of improving the efficiency of communication between mobile units that cannot communicate directly as a result of impediments to communication that block signals. In the technology disclosed in patent document 4, it is determined whether or not the state of communication for mobile units is good or poor based on detection results of an image processing apparatus that detects whether or not impediments to communication exist in the surroundings of the mobile unit. Hopping of information received by wireless transceivers is then controlled based on the results of this determination.
Patent Document 1: Unexamined Japanese Patent Application KOKAI Publication No. 2001-283381
Patent Document 2: Unexamined Japanese Patent Application KOKAI Publication No. 2001-119331
Patent Document 3: Unexamined Japanese Patent Application KOKAI Publication No. 2001-358641
Patent Document 4: Unexamined Japanese Patent Application KOKAI Publication No. 2005-51619